Invasive Species and the American West

May 29, 2019



NCEL Point of Contact

Kate Burgess
Conservation Program Manager


Most people have heard the term invasive species used before, though few understand what qualifies as an invasive species and the serious ecological threat they can pose. There are two main qualifications for a species to be considered an invasive species:

  • They must be a non-native species, meaning they are not originally from the area they are present in.
  • The species must cause, or be likely to cause, harm to the environment, the economy or to human health.

Invasive species often have one or more characteristics that give them an advantage over native species. Some common characteristics are high reproductive rates, few predators, adaptability to different conditions, and rapid growth. When an invasive species enters an area there’s a decline in native species populations as well as a decline in the overall health of the ecosystem. It’s estimated that invasive species have contributed to the decline of nearly 42% of threatened and endangered species in the US. The decline in ecosystem health can also lead to the loss of valuable land and ecosystem services (the direct and indirect benefits ecosystems provide us). Invasive species impact over 100 million acres of US land, and the national efforts to manage invasive species costs over $120 billion annually.Learn more about invasive species

Invasive Species in the West

Acknowledging the threat of invasive species, the Western Governors Association established the WGA Invasive Species Advisory Group in 2016 to provide technical assistance for ongoing efforts to tackle the issue. In doing so, they’ve created a list of the top 50 invasive species in the West.

To show how different types of invasive species can impact ecosystems, below are examples of a plant, mammal, and aquatic invasive species which have varying density populations in the West.

Cheatgrass (High Density in the West)

Cheatgrass is an annual invasive grass that is native to Europe and Eastern Asia. It was introduced to the US in the mid to late 1800s and is now found in 49 states, though it is particularly dominant in the intermountain west. Cheatgrass thrives in disturbed areas including construction sites, fires, floods, grazing and recreation and is hard to control once it becomes established. Due to cheatgrass being an annual plant, it blooms early in the spring and dies at the beginning of summer. The early bloom of cheatgrass gives it a competitive advantage over native plants, as does its short roots which absorb limited water and nutrients. In addition, the time it dies off aligns with fire season in the west, creating a serious wildfire hazard. Below is a map of cheatgrass locations in the west.

Feral Swine (Medium Density in the West)

Feral Swine is an invasive species that is found in at least 35 states and have a population of over 5 million. Wild hogs were originally brought over by early explorers and settlers in the 1500s as a source of food. Current feral swine populations come from these domesticated pigs that escaped from their enclosures and from Eurasian wild boars that were introduced in the 1900s for sport hunting. It’s estimated the management of feral swine costs nearly $1.5 billion a year. Feral swine cause serious damage through their feeding, rooting, trampling, and wallowing behaviors. They destroy crops, damage property, and harm ecosystems by causing soil erosion, vegetation loss and direct loss of wildlife. Finally, feral swine pose a serious health risk to people, pets, livestock, and wildlife. They can carry around 30 diseases and nearly 40 types of parasites.

While feral swine are not as widespread in the west as cheatgrass, their populations are projected to continue increasing due to their adaptability to different climates, high reproductive rates, translocation by humans, and their lack of natural predators. This means that feral swine will be an increasingly serious ecological issue for Western states to handle. The map below shows the distribution of feral swine in the US in 2018.

Mussels (Low Density in the West)

One of the most well known invasive species in the US are the quagga and zebra mussels. The mussels are native to Europe and were not found in the US until the 1980s where they were discovered in the Great Lakes Region. Since then, mussels have spread across the US and can now be found in 33 different states. These mussels are considered a serious threat because they lead to declines in native aquatic populations by stripping the food web. They also colonize on water intake pipes, leading to expensive maintenance costs, and on boats which can impact their steering, increase drag and clog the engines. When the mussels takeover beaches or rocklines on the shore they become a health risk due to their sharp shells. It’s estimated that the management of quagga and zebra mussels in the Great Lakes alone costs $500 million annually. While the map below shows that these mussels have not spread into the West as much as other invasives, they are considered a high priority for the Western Governors Association due to their potential to spread quickly and cause serious environmental, economic and human health problems. Quagga and zebra mussels have proven to be difficult to remove once established, leading Western states to prioritize preventing colonization before it begins.

What are states doing about these three invasive species?

Statewide management plans

  • Under the federal Aquatic Nuisance Species Prevention and Control Act states can get funding if they create a management plan that is approved by the Aquatic Nuisance Species Task Force. So far, 43 (40 state and 3 interstate) plans have been approved.
  • Some states have established task forces or departments whose specific goal is to manage invasive species within the state. Some examples are CaliforniaColorado, and Oregon.
  • The Western Regional Panel (WRP) for the Aquatic Nuisance Species Task Force created a Quagga-Zebra Mussel Action Plan for Western U.S. Waters

Specific actions to limit the introduction and spread

  • 47 states have established a list of invasive plants that cannot be sold, transported, or propagated within the state unless authorized by a permit. Only two states, Colorado and Connecticut, have included cheatgrass on that list.
  • Some states have established procedures for checking and cleaning water crafts to prevent the spread of mussels. Examples include Montana’s HB 553 and Nevada’s AB 82 both from 2015.

Efforts to eradicate existing populations

  • Oklahoma HB 1150: makes money available for projects to reduce feral swine populations.
  • Many fire management plans include controlling/eradicating wildfire fuels including cheatgrass.

new report from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services has found that the health of ecosystems is declining at rates that are unprecedented in human history.  The report found that 1 million plant and animal species are now threatened with extinction, and it cites invasive species as one of the five major drivers of this change.  Understanding this connection between invasive species and declining ecosystem health has led many states to take action against existing and potential invasives within their borders.